The present invention relates to a bookbinding machine which accumulates a sheet to make a sheet bundle and bonds a coversheet on the sheet bundle to form a square spine, and an image forming system having the bookbinding machine and an image forming apparatus main body.
For example, an image forming apparatus with electrophotographic method having high speed performance, a multi function and a network function is expanding its application as a printing apparatus by connecting with a large capacity sheet feeding apparatus and a large capacity stacker.
In case the image forming apparatus is used as a printing machine, one system can perform a job from printing to binding in a sequence of flow by connecting the bookbinding machine to bind prints.
In a bookbinding machine of Patent Document 1, a sheet on which an image is formed in an image forming apparatus is accumulated in a sheet accumulation section having an oblique sheet accumulation area to be shaped into a sheet bundle which configures a booklet, and the sheet bundle is conveyed to a jointing process section. Thereafter the sheet bundle is turned to be in vertical condition, and then application of an adhesive and jointing of coversheet are carried out for the sheet bundle in a vertical condition so as to form a book.
Also, as a bookbinding process it has been known that a sheet bundle in which printed sheets are accumulated is covered by a coversheet in U-shape to form a booklet.    Patent Document 1: Non-examined Patent Publication No. 2004-209869
In the bookbinding machine disclosed in Patent Document 1, a constant prescribed pressure is always applied onto the coversheet for pressing and forming each square spine, irrespective of changes of a thickness of the sheet bundle to be bound, and number and a basis weight of the sheet, as well as changes of size of the sheet for the booklet bound and a basis weight of the coversheet.
However, when the square spine is formed, if the pressure of a forming member is excessive, an amount of adhesive agent to run over increases and appearant quality of the square spine is deteriorated. Also, if the pressure of forming member is excessively low, the spine cannot be formed square enough, with the result that the appearance quality is deteriorated and strength of the booklet decreases.